The present invention relates generally to the field of teleconferencing devices, and is particularly adapted to providing an echo cancellation system for a teleconferencing device in which an instantaneous acoustic coupling is cancelled by use of a plurality of matched attenuators.
Adaptive finite impulse response (AFIR) filters are widely used for echo cancellation in long distance telephone circuits. See, e.g., O. A. Horna, "Echo Canceller Utilizing Pseudologarithmic Coding", NTC - 77, Conference Record, Vol. 1, pages 04:7-1 through 7-8. An AFIR filter produces an output signal which is a finite-in-time replication of a series of impulse signals at its input. In addition, the AFIR filter comprises means by which this output signal can be shifted in amplitude as a function of an error signal which is fed back to the filter. This error signal is the output of a comparator which compares the output of the AFIR filter with the echo signal component of a received signal. As referred to hereinafter, an "echo signal" is that portion of a received first signal which is reproduced in a transmitted second signal.
Experiments have shown that an AFIR echo canceller can also be used to cancel acoustic echo between the microphone and the loudspeaker of a teleconferencing device, as shown in FIG. 1. In FIG. 1, the received signal 1 is fed through amplifier 2 to both the input of the AFIR filter 14 and the input to the channel 101. The received signal is amplified through power amplifier 102 and is fed to the loudspeaker 21 of a teleconference device 200. The teleconference device operates in a well-known manner and employs a single microphone 22 mounted within its housing. Each conference participant must talk in turn through this single microphone.
The acoustic echo path for the teleconference device is the path over which the signal of the loudspeaker is received by the microphone. This acoustic echo path creates a disturbing echo signal which must be eliminated from the transmitted signal in order to maintain clear communications. The transmit signal from the teleconference device (hereinafter referred to as the "speech/echo signal") is fed by the microphone 22 through the microphone amplifier 103 to the positive input of the subtraction circuit 12 of the echo canceller 10. As previously described, the AFIR filter 14 emulates the echo path response and synthesizes an echo signal which is subtracted in the comparator from the speech/echo signal. The signal at the output of the echo canceller will be referred to as the "send-out" signal. The resulting send-out signal 1' is thus free of the signal from the loudspeaker 21, i.e. free of the acoustic echo. The adaptive operation of the AFIR filter is completed by use of the feedback signal 13; that is, the filter is adapted to overcome any residual echo remaining in the resulting send-out signal.
As previously stated, the above described echo cancellers are currently in use in long distance telephone circuits and other similar applications. However, the basic structure of the echo canceller must be modified in order to perform satisfactorily in a teleconference device application. Specifically, the basic echo canceller configuration as shown in FIG. 1 must be modified to compensate for the large dynamic range in the speech/echo signal. In telephone systems, the distance between the talker and the microphone is held nearly constant, and the acoustic echo path between the loudspeaker and the microphone is minimized by the "muffling effect" of the talker's head. These factors serve to limit the speech/echo signal level to a relatively narrow range of response. In teleconference devices, on the other hand, the distances between the acoustic inputs (e.g. the conference members sitting around a large table) and the microphone vary greatly, which produces a speech/echo signal which varies greatly in signal strength. Thus, means must be incorporated into the echo canceller for a teleconference device by which the dynamic range of the speech/echo signal is limited to the range of operation of the AFIR filter.
One of the methods used in the prior art to limit a signal to a relatively narrow range of response is to incorporate an automatic gain control (AGC) circuit into the signal path of the device in question. Referring to FIG. 2, if such a gain control circuit is placed in the receive-in path of the echo canceller (AGC 11 of FIG. 2), it will act as a level compressor and will protect the receive side of an echo canceller from overload. However, AGC 11 may also increase the gain for a weak signal; that is, it can boost the system's residual echo signal. Therefore, AGC 11 must be designed to have only a very limited operating range--it must protect from overload while being inactive for normal signal levels.
Another modification of the basic configuration of FIG. 1 which limits the dynamic range of the input signal to the echo canceller is to insert an AGC device in the speech/echo signal path at the input to the comparator of the echo canceller (AGC 29 of FIG. 2). However, this configuration will result in faulty operation of the AFIR filter of the echo canceller due to the non-linear operation of the AGC circuit. More particularly, with the AFIR filter fully adapted and operating at a steady state with no near-end speech, the true echo at the positive input of the comparator and emulated echo at the negative input are equal. When a person then talks into the microphone 22 of the teleconferencing device, AGC 29 will change the gain of the speech/echo signal. In changing the gain of the speech/echo signal, the echo path signal component will be adjusted along with the speech signal component. When the speech/echo signal is inputted to the comparator, the AFIR filter will output its predicted echo signal. These two signals, the echo replication signal from the AFIR filter and the echo path component of the gain-adjusted speech/echo signal, will now be unequal due to the fact that the emulated echo signal has not been gain-adjusted. Further, this inequality will be beyond the normal echo path adaptive parameters of the AFIR filter. The filter will sense this "double-talk" (or overload) situation, and will disable its adaptive operation in order to protect the stored response from contamination. The AFIR filter is thus unable to adapt to the new condition in the echo path, and instead of cancelling the echo, it can even generate an echo signal.